Thorp Air Command - T18.net

I have began to notice that the paint has popped off about 40% of the rivets (both top and bottom of wing) along the front spar of the inner wing section. This is happening on both wings. My plane does not have the second row of rivets on the inner wing spar. What should I do at this point? None of them are smoking rivets (yet).

What is the thickness of the inner wing skin? If .032, you can drill the second row and countersink for flush rivets. I know, the skin should be .040 for countersinking, but John Thorp told me I could get away with countersinking the .032 if I was careful to not go too deep. I countersank (?) to just the thickness of the skin and shaved any rivets that protruded. Served me well for many years.

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Bob Highley
N711SH
SN 835
KLAL

Are the rivets loose? Or did the paint just fall off of em........Dan

That's good news Bob. Thanks for the quick reply. I think I can handle that. I will check my skin thickness tomorrow.

Sorry for this grossly ignorant question, but should I use the appropriate length cherry max pop rivets with countersunk heads for the second row that I will install?

It's not an ignorant question Ryan. It seems like it might be an alternative. I'm thinking that if you used the slightly oversized CherryMax rivets, that I believe take a #27 hole (?), you might be able to get a full countersink for their heads in .032" material, since they are slightly shallower. They might not pull down quite as tightly... I'm not certain, but If I used them I think I'd use an electric or pneumatic puller to really get a good set. It would be nice to not have to open up the wing too much, but I guess you might have to anyway. Maybe not, with the proper deburing tool.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I get the feeling cherry max pop rivets are not the recommended repair method. Is the proper way to fix this accomplished by opening up the wing, and shooting/bucking solid rivets, or are cherry max pop rivets an acceptable repair also? I want to do the right thing here, but if I don't have to open up the wing, then I would rather not. What are your repair suggestions?

I would bite the bullet and buck rivets. Also check and make sure while your are at it that all the mods and parts have been installed. drawing 322 adds additional rivets to the leading edge rib. Check and see if the 3/4 angles have been installed on the spar

I used 100's of CM rivets constructing my AC . More on the wings than anywhere else . CM rivets were used as I was the sole "riveter" and some AN rivets were impossible for me to install single handed . This is quote from the Cherry Aerospace web site..."The Cherry Max rivet is the most reliable, high strength structural fastener with visual inspectability in the world today . It features the "safe lock" locking collar for more joint integrity ."

I was able to purchase lots of 1000 CM rivets off ebay from a fellow in Canada for about $10.00 per bag of 100 making them very cost effective . No need to tear the wing apart to buck rivets .

RB

I tend to agree that solid AD rivets are the preferred choice for most, but Rich is exactly right. I didn't mean to imply that the Cherry Max rivets were a poor choice for this particular job, Ryan. I would personally consider them. There is a certain limited amount of controversy over their use in "structural" applications with aircraft. There have been isolated incidents of issues that, in my opinion, were sometimes possibly caused by poor practices - preparation of holes, installation, etc., that resulted in multiple cracks developing in the aluminum at rivet holes in one instance of a fatal helicopter accident . This has led some to make blanket statements about their suitability for certain applications. We all know about the legal system in this country.

Keep in mind that the vast majority of T-18s flying today do not have the double row of rivets on the main center section spar (mine included). You are only going to improve yours if you add a row of Cherry Max rivets, IMHO. There will be vastly less work, and drilling out a bunch of rivets, opening up the wing, & then re-riveting the now less than perfect holes would be a consideration for me. Also, as I mentioned you could use the slightly larger diameter (1/64") CR3242 (MS 7885/7) rivets, which would make the necessary countersink depth slightly less - a consideration with the .032 skin thickness. You could even consider that they make another rivet - I don't know the number off the top of my head - that has a smaller head, requiring probably even less countersink depth (designed for thinner materials like that). I hope this helps you a little in your decision making.

These are all just some random thoughts, and my personal opinions only. You'll do what is best in your own mind of course.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I am seeking thoughts from everyone about the best path forward here. So I am truly grateful you (John), Rich, Bill and Bob have spoken up to give me your opinions, thoughts, and recommendations. That is exactly what I am looking for.

I use you guys as my frame of reference to determine what is acceptable or not. Just because I can drill a hole and smash a rivet (any monkey can), I don't know anything about major structural repair/maintenance, so I dont want to assume anything when dealing with the main spar.

I'll just throw in my 2 cents as a similar subject has been asked of me in the past. If you drill a hole in the spar, that hole needs to be filled properly to be structurally sound. Drilling a hole does not make the spar weaker. If you fill that hole with a properly sized and constructed rivet, it is just as sound as if you never drilled that hole.
Ryan, if it bothers you, I would drill the additional holes that RB and John mention and use CM rivets. To be perfectly honest, my Thorp is doing the same thing and I'm just leaving it alone.
Cubes

Now that the paint has popped of the rivets, I thought this was an indicator the rivets had become loose. Based on seeing the paint popping indicator, I thought if I didn't reinforce the single row of rivets with the second row of rivets, that this single row of rivets would continue to become more loose over time creating a worse situation than I have now. Is that incorrect?

I want to leave it alone, but what are the negative effects (if any) of doing so?

I'm not the genius on this block. But I see no evidence of working on mine. That means they are not getting loose. Therefore I'm not touching them. If I do, I'll CM them. I am assuming there was an issue with paint prep.
Yours could be totally different.
Cubes

Talked to our resident airframe expert at the airport this morning. His comment was use cherry locks of the correct length and not cherry max.

Not sure what type of equipment you need to set the Cherry Lock rivets , but the CM rivets can be set using a mark one mod one hand rivet puller (get the one with the rotating head made in the USA) or I used a pneumatic rivet puller from HF for about $40.00 . It worked setting the rivets according to the CM site regarding correct stem length and formed rivet head . The only draw back to the HF unit is that after setting a number of rivets the oil gets low in the unit and it has to be refilled . Not a big deal . Not sure if these rivet pullers will work with Cherry Lock ? Puzzled as to the structural difference between CL and CM rivets ? CM rivets can be found thru most aviation supply outfits . Not sure about CL ?

RB